A seat belt system for a vehicle typically has a seat belt retractor that serves to retract the belt into a housing. The belt is wound upon a spool in the housing. When the belt is drawn out or protracted, the spool winds a retraction spring, which later retracts the unused portion of the belt onto the spool or withdraws the belt into a housing when not in use.
In a crash the seat belt retractor has a lock that limits the extension of the seat belt from the housing. The lock may be actuated by an inertial sensor, which responds to changes in vehicle speed that occur during the crash. When a large deceleration is detected, the inertial sensor triggers the lock of the seat belt retractor to secure the seat belt in place during the crash.
In a locked condition, the belt restrains the vehicle occupant from moving forward during a crash. Although the seat belt has some give, the restraining force on the vehicle occupant can be significant. To reduce this force, manufacturers may use a torsion bar to absorb energy from the forward movement of the vehicle occupant in a controlled manner. Generally, the spool is mounted on the torsion bar, which is mechanically linked to the spool. During a crash, the torsion bar twists and deforms as the spool rotates from the protraction of the seat belt from the housing. The deformation of the torsion bar absorbs energy from the seat belt. Consequently, the vehicle occupant is gradually slowed rather than suddenly stopped during the crash.
The weight of the vehicle occupant can affect the rate by which the vehicle occupant is slowed by the restraining force of the seat belt. Heavier vehicle occupants require greater restraining force than lighter vehicle occupants. It is desirable to use a higher rate of energy absorption for a heavy vehicle occupant than for a light vehicle occupant.
Recently, manufacturers have commenced using seat belt retractors that absorb energy at different rates to accommodate differently weighing vehicle occupants. For example, when a small vehicle occupant is seated in the vehicle the seat belt retractor is set at a low rate of energy absorption. Thus, the lower weighing vehicle occupant is restrained with less restraining force than a heavier vehicle occupant. On the other hand, for a heavier vehicle occupant, a higher energy absorbing rate is used to slow the vehicle occupant using a greater restraining force. A vehicle occupant weighing somewhere in the middle may require a combination of restraining force rates during the course of the crash. Thus, a vehicle occupant may receive a restraining force that accommodates his weight.
There are incidents where a vehicle experiences more than one crash. It is generally desirable to continue to absorb energy at the same high rate for the heavy weight vehicle occupant in a second crash. However, for a lightweight occupant, it is preferable to absorb energy from the seat belt spool initially at a low rate for the first crash, then at the higher rate for the second crash. For a middleweight vehicle occupant, it is desirable to absorb energy at a high rate and then a low rate for the initial crash. For the second crash, a high rate of energy absorption is preferred. Conventional seat belt retractors do not have such a feature.